[SPARK-13976][SQL] do not remove sub-queries added by user when generate SQL

## What changes were proposed in this pull request?

We haven't figured out the corrected logical to add sub-queries yet, so we should not clear all sub-queries before generate SQL. This PR changed the logic to only remove sub-queries above table relation.

an example for this bug, original SQL: `SELECT a FROM (SELECT a FROM tbl) t WHERE a = 1`
before this PR, we will generate:
```
SELECT attr_1 AS a FROM
  SELECT attr_1 FROM (
    SELECT a AS attr_1 FROM tbl
  ) AS sub_q0
  WHERE attr_1 = 1
```
We missed a sub-query and this SQL string is illegal.

After this PR, we will generate:
```
SELECT attr_1 AS a FROM (
  SELECT attr_1 FROM (
    SELECT a AS attr_1 FROM tbl
  ) AS sub_q0
  WHERE attr_1 = 1
) AS t
```

TODO: for long term, we should find a way to add sub-queries correctly, so that arbitrary logical plans can be converted to SQL string.

## How was this patch tested?

`LogicalPlanToSQLSuite`

Author: Wenchen Fan <wenchen@databricks.com>

Closes #11786 from cloud-fan/bug-fix.